1. Field of the Invention
The invention relates to a fixing device for fixing an image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, a printer, a facsimile or an MFP having a function of these devices, and the like, a copied matter and a recorded matter can be obtained by heating and fixing an unfixed image transferred onto a recording medium such as a sheet and carried thereby.
In the fixing process, a developer, in particular, toner included in an unfixed image is melted and softened and penetrated into a recording medium by heating the unfixed image carried by the recording medium while sandwiching and conveying the recording medium by a fixing member and a pressing member, thereby the toner is fixed onto the recording medium.
Further, when the fixing member is heated to a predetermined temperature by a heat source, if a heat time up to the predetermined temperature is sufficiently short, a preheat process in a stand-by state can be omitted, and thus the amount of energy to be consumed can be greatly reduced. To achieve the energy saving effect, a member having a low heat capacity such as a thin roller, a belt, and the like, which is composed of a metal base member and an elastic rubber layer, is widely used as the fixing member. Further, rapid heating is performed by using a ceramic heater, an IH system having high heating efficiency, and the like as the heat source in addition to a halogen heater for heating the fixing member by radiant heat. Fixing devices having such configurations are disclosed in, for example, Japanese Patent Application Laid-open No. 2007-79040, Japanese Patent Application Laid-open No. 2010-32625, Japanese Patent Application Laid-open No. 2007-334205 and Japanese Patent Application Laid-open No. 2008-129517.
Among the fixing devices, in a fixing device configured to stretch a fixing belt by a fixing roller and a heating roller, a fixing device that is heated by an IH system, a fixing device for locally heating a fixing member by deviating the position where a built-in halogen heater is installed (partial heat system), a heat region that is heated by a heat source is positionally different from a fixing nip portion. Accordingly, even in a fixing member that is heated to a relatively high temperature in a heat region, during the image fixing operation, the temperature of the fixing member is not abnormally increased because a recording medium passing through a fixing nip portion takes heat from the fixing member. On the other hand, in a state that the rotation of the fixing member is stopped as in a case that the image fixing operation has been finished, even if a power supply to the heat source is stopped, the fixing member may be placed in an excessively increased temperature state by the excess heat (remaining heat) of the heat source (when sheets are continuously fed, a larger amount of heat is accumulated in a fixing device). Otherwise, even when excess heat in a heat source is not a so serious problem, the temperature of a surface of a fixing member may be increased by the excess heat of a reflector, a stay, and inside air whose temperature has been increased after the rotation of the fixing member is stopped. Further, when a heat region is away from a fixing nip portion, since the heat region is partially heated to a relatively high temperature to secure an amount of heat necessary to fixing at the time a belt portion heated in the heat region moves to the fixing nip portion, unless the heat is taken from the belt portion, the belt portion is damaged. In particular, in a fixing device having a fixing member whose thickness is further reduced to have a low heat capacity in order to reduce a warm-up time and energy to be consumed, there is a tendency that the problem is likely to occur in the fixing member.
In a partially heating fixing device, when an image fixing operation is finished and a fixing device is stopped, a pressing member takes the heat from a fixing member at the region of a fixing nip portion in contact with the pressing member. However, in the other region, in particular, in a region up to a fixing nip portion including the heat region, since heat remains stored because heat moves relatively slowly in a circumferential direction, the region is particularly thermally expanded. When a difference of a thermal expansion amount occurs between a high temperature region and a low temperature region because a temperature difference is large in a circumferential direction of the fixing member and the difference becomes excessively large, kink (plastic concaved crush formed in the fixing member) is generated in a central portion on a high temperature side. The generation of kink causes an abnormal image and further breaks the fixing member.
Ordinarily, since a temperature sensor is disposed in the vicinity of a heat source, when a temperature increase equal to or larger than a predetermined value occurs, the fixing member is thermally expanded in its entirety by being rotated or by taking heat from the fixing member in its entirety by a pressing member, thereby local expansion is prevented by making the temperature difference of the fixing member in a circumferential direction equal to or less than a prescribed value, and the generation of kink is avoided.
However, recently, since energy saving is emphasized, there are more cases that after an image forming operation is finished, a ready/stand-by state and a low power state are made very short and an operation is promptly shifted to a so-called sleep state in which power of the entire of an image forming apparatus is stopped, and then a power supply is resumed to the entire of the image forming apparatus when a signal is input from the outside or an operation panel. Further, there is also an image forming apparatus of a type that is provided with a sleep mode shift button and can be forcibly shifted to a sleep mode by a user. Note that a state called “off mode” in a copying machine is the same state as the “sleep mode”, and the following explanation will be made using an expression “sleep”.
In the sleep state, since only restart power is consumed, the power consumption of various devices can be reduced. However, after the operation is shifted to the sleep state as described above, an excessive temperature increase cannot be prevented because a temperature sensor cannot detect a temperature and a fixing member cannot be rotated. In particular, when sheets of paper are continuously fed and a larger amount of heat is accumulated in a fixing device, a serious problem of overshoot occurs.
Therefore, there is a need for a fixing device and an image forming apparatus capable of suppressing kink from being generated, to execute a shift to sleep state when it is possible to enter to a sleep state instantly after the completion of an image forming operation, and to achieve energy saving.